In recent years, there have appeared many mobile or portable electronic devices such as cell-phones including smartphones, potable personal computers, PDAs, and portable game machines. Sealed batteries are used as driving power sources for these. Among sealed batteries, non-aqueous electrolyte secondary batteries capable of repeated charge and discharge are particularly widely used because of their lightweight and high capacity.
If a sealed battery is overcharged due to misuse, charger malfunction, or the like, the pressure in the battery increases with the decomposition of electrolyte. If the sealed battery continues to be overcharged, owing to the protrusion of a sealing body of the sealed battery, the breakage of an outer can, or the like, electrolyte spatters around the sealed battery and damages an electronic device housing the sealed battery. So, the sealed battery is provided with a safety mechanism that cuts off current when the pressure in the battery reaches a predetermined value.
PTL 1 discloses a safety mechanism that, when the pressure in a battery reaches a predetermined value, cuts off current by breaking a welded part between a partition wall in a sealing body and a welded plate connected to a conductive lead of an electrode plate. PTL 2 discloses a safety mechanism that, when the pressure in a battery reaches a predetermined value, cuts off current by breaking not a welded part between an upper metal foil and a lower metal foil in a sealing body but a thin, easily-breakable part provided in the lower metal foil. PTL 3 discloses laser welding an upper metal foil and a lower metal foil in a sealing body.